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Optical bistability for three-level V-type atomic medium in ring cavity is examined in the presence of squeezed vacuum field within and without plane wave approximations. Squeezed Vacuum parameters affects the steady state behavior of the system. Gaussian field feature causes a significant change in the steady state behavior.

Decay parameter of coherence and population inversion are calculated from the master equation of a two-level atom tunneling through a squeezed vacuum. Using those parameters, the timescales for decoherence and Zeno effect are calculated in the weak measurement scheme. By comparing those timescales, a certain condition on the relevant experimental parameters has been found for sustainable coherent dynamics.

We observe the polarization squeezing in the mixture of a two mode squeezed vacuum and a simple coherent light through a linear polarization beam splitter. Squeezed vacuum not being squeezed in polarization, generates polarization squeezed light when superposed with coherent light. All the three Stokes parameters of the light produced on the output port of polarization beam splitter are found to be squeezed and squeezing factor also depends upon the parameters of coherent light.

The use of the coherent and squeezed state formalisms of quantum optics for a minimally coupled nonclassical inflaton in the FRW metric is studied, in the semiclassical theory of gravity. The leading order solutions to the semiclassical Einstein equations in the coherent, squeezed and squeezed vacuum states are obtained perturbatively and exhibit power law expansion behavior. The validity of the semiclassical theory is examined in the squeezed vacuum state in the oscillatory phase of the inflaton. The semiclassical theory in the oscillatory phase of the nonclassical inflaton holds only if the associated squeezing parameter is much smaller than unity. Quantum fluctuations of the inflaton are also examined in the coherent and squeezed state formalisms.

The interaction between the pair of cold two-level atoms and the single-mode cavity field is investigated. The two-level atoms in the pair are supposed to be indistinguishable. This problem generalizes the two-photon Jaynes-Cummings model of a single two-level atom interacting with the squeezed vacuum. The model of the pair of indistinguishable two-level atoms is equivalent to the problem of the equidistant three-level radiator with equal dipole moment matrix transition elements between the adjacent energy levels. Supposing that at the initial moment the field is in the squeezed vacuum state we obtain the exact analytical solution for the atom-field state-vector. By using this solution the quantum-statistical and squeezing properties of the radiation field are investigated. The obtained results are compared with those for the single two-level atom system. We observe that in the model of the pair of cold two-level atoms the exact periodicity of the squeezing revivals is violated by the analogy with the single two-level atom one.